J. Pederson

An evaluation of the regional acid deposition model surface module for ozone uptake at three sites in the San Joaquin Valley of California

Plants and soils act as major sinks for the destruction of tropospheric ozone, especially during daylight hours when plant stomata open and are thought to provide the dominant pathway for the uptake of ozone. The present study, part of the California Ozone Deposition Experiment, compares predictions of the regional acid deposition model ozone surface conductance module with surface conductance data derived from eddy covariance measurements of ozone flux taken at a grape, a cotton, and a grassland site in the San Joaquin Valley of California during the summer of 1991. Results indicate that the model (which was developed to provide long-term large-area estimates for the eastern United States) significantly overpredicts the surface conductance at all times of the day for at least two important types of plant cover of the San Joaquin Valley and that it incorrectly partitions the ozone flux between transpiring and nontranspiring components of the surface at the third site. Consequently, the model either overpredicts or inaccurately represents the observed deposition velocities. Other results indicate that the presence of dew does not reduce the rate of ozone deposition, contradicting to model assumptions, and that model assumptions involving the dependency of stomata upon environmental temperature are unnecessary. The effects of measurement errors and biases, arising from the presence of the roughness sublayer and possible photochemical reactions, are also discussed. A simpler model for ozone surface deposition (at least for the San Joaquin Valley) is proposed and evaluated.

California ozone deposition experiment: Methods, results, and opportunities

Abstract The California Ozone Deposition Experiment (CODE) is a program of observations and modeling to improve estimates of the rate of removal of tropospheric ozone at the earths surface used in grid-based photochemical models of ozone production, transport, and removal. The purpose of CODE is to test, diagnose and improve treatment of dry deposition of ozone and other gaseous species. CODE supports a larger air quality measurement and modeling effort comprised of the San Joaquin Valley Air Quality Study (SJVAQS) and Atmospheric-Utilities Signatures: Predictions and Experiments (AUSPEX) joined as SJVAQS/AUSPEX Regional Model Adaptation Project (SARMAP). However, the CODE data are also applicable to a variety of boundary layer and turbulence problems. This paper describes the field methods and data collected during summer (10 July through 6 August) of 1991 in the San Joaquin Valley (SJV) of California and introduces several related papers. General comparisons and conclusions from all the participants are summarized. The core elements of the CODE field effort consisted of a research aircraft for spatial coverage and three ground sites located in a cotton field, grape vineyard, and very dry (senescent) annual grassland. A major portion of the SJV is represented by these three vegetation types. The eddy covariance method is used to compute the vertical fluxes of ozone, carbon dioxide, water vapor, sensible heat and momentum. For the first half of the study period, flights were made mainly for comparison with tower-based fluxes. Subsequent flights were over other vegetation types and to conduct special studies. In addition to the vertical fluxes, the ground-site data include individual leaf measurements of stomatal conductance, radiative leaf temperature, wetness of surrogate leaves, soil temperature profiles and heat flux, soil composition and water content, mean nitrogen oxide and ozone concentrations, solar and net radiation, photosynthetically active radiation, and vertical profiles of wind, temperature, ozone and water vapor. Aircraft data also include reflected short-wave radiation, surface greenness index and radiative surface temperature. Several factors simplify analyses: a nearly constant synoptic situation, lack of cloud cover, low-level (30 m) flights and land use characterized by extensive homogeneous areas with well defined interfaces. Repeated five-km aircraft runs, necessary for a representative flux calculation, were commonly made over a single crop type. In addition, a partial (60%) solar eclipse on 11 July provides an opportunity to examine the influence of light intensity upon the plant-atmosphere exchange of carbon dioxide and ozone via stomatal activity.

Effects of stomatal conductance and surface wetness on ozone deposition in field-grown grape

Surface deposition is an important sink that removes ozone from polluted air basins, and leads to crop damage and ecosystem decline. Physiological and physical processes controlling deposition to vegetated surfaces are incompletely understood. We investigate the relationship between ozone flux to trellised grape, F, and canopy stomatal conductance to ozone, gc, under dew-wetted and dry conditions. Empirically measured stomatal conductance was scaled to gc using empirical measurements of leaf area index, L, single leaf stomatal response to photon flux density, I, and bulk canopy radiation extinction coefficient, K. Leaf wetness was determined with surrogate leaves covered with electrical impedance grids. Deposition velocity, Vd, and surface conductance, gsurf, were positively and highly significantly related to gc. Surface wetness substantially increased Vd and gsurf. Under all conditions, gc < gsurf, suggesting a significant non-stomatal (residual) pathway for ozone deposition, gr. This residual term, gr, was increased under wet conditions by a constant amount over the full range of gc. Expected errors of ± 20% in the single leaf model, in L, or in K, did not influence these conclusions. We conclude that Vd and gsurf were dominated by c, which may be used effectively to predict ozone deposition to physiologically active vegetated surfaces. Dew formation enhanced ozone deposition to the hypostomatous leaves of this grape canopy by a non-stomatal pathway.

Surface conductances for ozone uptake derived from aircraft eddy correlation data

Abstract Plants and soils act as major sinks for tropospheric ozone, especially during daylight hours when plant stomata are thought to provide the dominant pathway for ozone uptake. The present study, as part of the larger California Ozone Deposition Experiment, uses aircraft eddy covariance measurements taken during the summer of 1991 in the San Joaquin Valley of California to estimate the surface conductance for ozone uptake. To explore for possible sources of discrepancies between the aircraft-derived and tower-based surface conductances a comparison is first made between tower-based fluxes and aircraft fluxes at three tower-based sites. On the average the momentum and surface energy fluxes (sensible and latent heat) observed between 30 and 33 m altitude with an aircraft agreed to within ± 10% with simultaneously measured tower-based fluxes (observed between 4 and 10 m at a vineyard, a cotton and a grassland site). However, comparisons of the aircraft- and tower-based ozone fluxes indicate that between about 4 and 33 m there is an average loss of ozone flux with height of about 18%. It is suggested that either (or both) soil NO emissions or entrainment of ozone free air at the top of the mixed layer may be responsible for this relatively larger discrepancy in the ozone fluxes. Nevertheless, in spite of any relatively larger uncertainties associated with the ozone flux, the tower-based and aircraft-based conductances are in good agreement. The aircraft-derived conductances display a similar magnitude and range of variation as the tower-based conductances and the regression coefficient between the two sets of conductances is 0.9 ± 0.08. Therefore, results from this study suggest that the aircraft can be used to estimate surface conductances of ozone deposition; however, these conductances are subject to large uncertainties.

High-global warming potential F-gas emissions in California: comparison of ambient-based versus inventory-based emission estimates, and implications of refined estimates.

To provide information for greenhouse gas reduction policies, the California Air Resources Board (CARB) inventories annual emissions of high-global-warming potential (GWP) fluorinated gases, the fastest growing sector of greenhouse gas (GHG) emissions globally. Baseline 2008 F-gas emissions estimates for selected chlorofluorocarbons (CFC-12), hydrochlorofluorocarbons (HCFC-22), and hydrofluorocarbons (HFC-134a) made with an inventory-based methodology were compared to emissions estimates made by ambient-based measurements. Significant discrepancies were found, with the inventory-based emissions methodology resulting in a systematic 42% under-estimation of CFC-12 emissions from older refrigeration equipment and older vehicles, and a systematic 114% overestimation of emissions for HFC-134a, a refrigerant substitute for phased-out CFCs. Initial, inventory-based estimates for all F-gas emissions had assumed that equipment is no longer in service once it reaches its average lifetime of use. Revised emission estimates using improved models for equipment age at end-of-life, inventories, and leak rates specific to California resulted in F-gas emissions estimates in closer agreement to ambient-based measurements. The discrepancies between inventory-based estimates and ambient-based measurements were reduced from -42% to -6% for CFC-12, and from +114% to +9% for HFC-134a.